<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>For循环优化</title>
</head>

<body>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>


    <script>
        var aBtns = document.getElementsByClassName('btn')

        for (var i = 0; i < aBtns.length; i++) {
            console.log(i)
        }
        
        // 优化后，避免了aBtns的长度计算
        for (var i = 0, len = aBtns.length; i < len; i++) {
            console.log(i)
        }
    </script>
</body>

</html>